1. Field of the Invention
The present invention relates to boiling water reactor (BWR) fuel assemblies and more particularly to a BWR fuel assembly capable of reversible or inverted operation.
2. Description of the Prior Art
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of nuclear fuel rods are grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical nuclear fuel assembly may be formed by an 8.times.8 array of spaced-apart, elongated rods supported between upper and lower tie plates. Examples of such typical nuclear fuel assemblies are depicted in and described in U.S. Pat. Nos. 3,350,275; 3,466,226; 3,802,995. In a typical BWR nuclear fuel assembly having an 8.times.8 rod array, the sixty four rods that form the 8.times.8 array may be either sixty four fuel rods or may have one or more nonfueled, water moderator rods with the remaining rods being fuel rods.
A common problem in BWR reactors is the high fuel costs associated with the typical BWR fuel cycle. In BWR fuel assemblies, the fuel near the bottom of the fuel assemblies is "burned up" at a faster rate than the fuel near the top of the assembly. This is because in a BWR, the power spectrum is skewed towards the bottom of the fuel assembly. This skewing is a result of the change in void fraction in the fuel assembly from the top to the bottom. Boiling of coolant typically commences about one quarter of the way from the bottom of the fuel assembly. From that point upwards, the void fraction increases to as high as sixty to seventy percent at the top of the fuel assembly. Since the coolant acts as a neutron moderator, thermalizing fast neutrons, the low H/U ratio at the top of the core results in a situation where the top of the core is less reactive than the bottom and therefore generates less power.
In typical BWR reactors, the control rods enter the core from the bottom for the purpose of limiting the skew and power distribution. However, even with control rod insertion, more power is produced and more burn up takes place at the bottom of the core. Thus, the fuel at the lower end of the core, where the water density is high, is more completely burned than the fuel at the top of the core. This results in less than optimum burn up of fuel and overall higher fuel costs.
In copending, commonly assigned U.S. application Ser. No. 609,250 filed currently herewith, an improved (PWR) pressurized water reactor fuel assembly is disclosed having fuel rods with plenum zones for fission gases where some of the fuel rods have plenum zones at the bottom of the rod for the purpose of enhanced fuel utilization and reduced neutron leakage.
In U.K. Pat. No. 923,633 there is disclosed a method of continuously charging a BWR in order to achieve a more uniform burn up of fuel. The method involves dividing the fuel assembly into axial and radial zones of differing mean burn up rate and then continuously transporting the fuel rods from zone to zone according to a complex schedule.
Itoh et al and U.S. Pat. No. 4,119,489 discloses a nuclear fuel assembly design in which the tubular channel members surrounding the fuel assemblies is removably supported by the upper tie plate so that the channel member may be removed, turned upside down, and then reinserted over the fuel assemblies so as to minimize the effects of flow channel deformation. In the asssembly of Itoh et al, the fuel assemblies themselves are not moved nor are they reversible.
Thus, the prior art fails to teach a BWR fuel assembly which is compatible with and which can be substituted for original fuel during refueling operations and which can achieve more complete burn up of the fuel.